1. Field of the Invention
This invention generally relates to a reading apparatus for optically reading an oiginal document, and, in particular, to a color original reading apparatus for reading a color original using a plurality of line sensors.
2. Description of the Prior Art
In a typical prior art system for optically reading a color original, light having image information reflecting from the color original, which is illuminated by a light source, is passed through a focusing lens and the light is then separated into two or more beams which are passed through respective color filters before impinging on respective image sensors. In the system of this type, there are those which use a dichroic mirror or dichroic prism having a characteristic of reflecting light of a particular color component and allowing the light of the other color components to pass therethrough, in which case the light having image information is subjected to beam splitting and color separation at the same time, and there are those which use a half mirror for splitting the light beam into two light beams and a color separation filter for each of the split light beams for allowing light of a particular color component to pass therethrough.
However, with such a structure, when the light having image information and being convergent after passing through the focusing lens is passed through the above-described mirror or prism, the degree of astigmatism increases so that the image formed on a line image sensor tends to be defocused. For this reason, in order to obtain a focused image of high quality, it is required to use a focusing lens having a characteristic to compensate for the astigmatism due to the provision of the above-described mirror or prism. In addition, such components as dichroic mirrors and prisms and half-mirrors are relatively expensive, so that the prior art systems tend to become high at cost.